This invention relates to frequency stabilized lasers and their application to interferometry, including, for example, distance measuring interferometry.
Interferometers exploit the wave nature of light to measure properties such as distance, flatness, and refractive index. For example, a displacement measuring interferometer (DMI) monitors a change in the position of a measurement object relative to a reference object based on an optical interference signal formed by overlapping and interfering a measurement beam reflected from the measurement object with a reference beam reflected from the reference object. For many applications, a laser is used to provide light for the interferometric measurement and the accuracy of the measurement is typically a small fraction of the wavelength of the light generated by the laser for the measurement (e.g., about, or even less than, 1 nm). To provide such accuracy, however, the wavelength (or corresponding optical frequency) of the generated light should be stable during the interferometric measurements.
The optical frequencies of light generated by a laser depend on the resonant modes supported by the laser cavity (which define resonant frequencies), the frequency-dependent gain of the gain medium in the laser (known as the gain curve), and the frequency-dependent losses of the laser cavity. The frequencies that lase are those resonant frequencies for which gain from the gain medium exceeds loss from the laser cavity.
Variable conditions both inside and outside the laser cavity can cause changes in the cavity length, thereby changing the resonant frequencies and the lasing frequencies therein. In addition, because the gain from the gain medium varies with frequency (the peak gain corresponding to a quantum transition frequency), the relative intensity of the laser output at each lasing frequency changes when the lasing frequency changes. As a result, changes in the relative intensity of the laser output at different lasing frequencies can be measured to monitor changes in the lasing frequencies themselves. Therefore, the lasing frequencies can be stabilized by adjusting the laser cavity length in response to the measured changes in the relative intensities.
In general, in one aspect, the invention features a frequency stabilized laser system including a laser source, a first detector, a second detectors, and a circuit. The laser source has an adjustable cavity length and, during operation, produces a control beam including two frequency components having different polarizations. The first detector absorbs a portion of the control beam, reflects the remaining portion of the control beam, and generates a first signal proportional to the intensity of the absorbed portion of the control beam. The second detector absorbs a portion of the reflected beam and generates a second signal proportional to the intensity of the absorbed portion of the reflected beam. The circuit receives the first and second signals from the detectors and generates a control signal based on the first and second signals that controls the adjustable cavity length of the laser source.
Embodiments of the frequency stabilized laser system can include any of the following features.
The gain of the first detector used to produce the first signal can differ from the gain of the second detector used to produce the second signal, and the control signal can be based on the difference between the first and second signals. Alternatively, or in addition, the circuit can generate the control signal based on a weighted difference between the first and second signals.
The control beam can contact the first detector at a first angle of incidence and the reflected beam can contact the second detector at a second angle of incidence such that the relative intensities of the two frequency components in the absorbed portion of the control beam differs from the relative intensities of the two frequency components in the absorbed portion of the reflected beam. For example, the control beam can contact the first detector at a first angle of incidence in a range between 5xc2x0 and 85xc2x0. Also, the first angle of incidence can be substantially equal to the second angle of incidence. The first detector, second detector, or each of the first and second detectors can be a silicon detector that does not have an antireflection coating.
The second detector can reflect a portion of the reflected beam to produce a reference beam. The control beam contacts the first detector at a first angle of incidence and the reflected beam contacts the second detector at a second angle of incidence such that the reference beam includes the two frequency components. The frequency stabilized laser system can further include a fiber optic coupler positioned to receive the reference beam. Alternatively, or in addition, at least one of the first and second detectors can have a bandwidth sufficient to resolve an optical interference signal at a frequency that is equal to the difference frequency of the two frequency components.
The different polarizations in the control beam produced by the laser source can be orthogonal elliptical polarizations. The laser source can include a Zeeman-split laser that produces the control beam including the two frequency components having the different polarizations. The Zeeman-split laser can include a birefringent element that causes the different polarizations to be different elliptical polarizations. For example, the birefringent element can be a coated cavity mirror. Alternatively, or in addition, the laser source can further include a birefringent element positioned to receive an input beam from the Zeeman-split laser and produce the control beam, wherein the birefringent element causes the different polarizations of the control beam to have different elliptical polarizations. The control beam can be derived from leakage through one of the cavity mirrors in the laser source.
The laser source can also produce, during operation, a measurement beam including the two frequency components. The frequency components of the measurement beam are stabilized by the interaction between the circuit and the adjustable cavity length. The laser source can include a transducer to adjust the cavity length of the laser source. For example, the laser source can include a gas tube defining the cavity length and the transducer can be a heating coil in thermal contact with the gas tube.
Many embodiments of the frequency stabilized laser system have a relatively small number of optics. For example, embodiments of the system can have no intervening optics contacting the control beam between the laser source and the first detector and/or no intervening optics contacting the reflected beam between the first and second detectors.
In another aspect, the invention features an interferometry system including the frequency stabilized laser system described above and an interferometer. The interferometer receives a measurement beam produced by the laser system, directs a portion of the measurement beam along a path contacting a measurement object, and recombines the portion with a remaining portion of the measurement beam to produce an output beam. The output beam has a phase indicative of changes in an optical path length to the measurement object.
In another aspect, the invention features an interferometry system including the frequency stabilized laser system described above, an interferometer, and a detection system. During operation, the interferometer receives a measurement beam produced by the laser system, directs a portion of the measurement beam along a path contacting a measurement object, and recombines the portion with a remaining portion of the measurement beam to produce an output beam. The detection system receives the output beam and a reference beam produced by the laser system, measures frequencies of the output and reference beams, and determines changes in an optical path length to the measurement object based on the measured frequencies.
In general, in another aspect, the invention features a method for stabilizing the frequency output of a laser source producing a control beam including two frequency components having different polarizations, the laser source having an adjustable cavity length. The method includes: directing the control beam to a first detector that absorbs a portion of the control beam, reflects the remaining portion of the control beam, and measures an intensity of the absorbed portion of the control beam; directing the reflected beam to a second detector that absorbs a portion of the reflected beam and measures an intensity of the absorbed portion of the reflected beam; and adjusting the cavity length of the laser source based on a control signal derived from the intensities measured by the first and second detectors.
Embodiments of the method can include any of the following features. The method can further include determining the control signal by scaling the intensities measured by the first and second detectors such that the difference between the scaled intensities is indicative of the relative intensities of the two frequency components produced by the laser source. The method can further include resolving an interference signal in the measured intensity of at least one of the detectors corresponding to the difference frequency of the two frequency components produced by the laser source. Alternatively, or in addition, the second detector can reflect a portion of the reflected beam to define a reference beam, and the method can further include measuring an optical interference signal in a reference beam corresponding to the difference frequency of the two frequency components produced by the laser source.
In another aspect, the invention features an interferometry method that includes stabilizing the frequency output of a laser source as described above and using the stabilized output to make interferometric measurements.
The invention has many advantages. For example, the frequency stabilized laser system is inexpensive and compact, not requiring expensive optics such as quarter wave plates or polarizing beam splitters for frequency stabilization. The laser system can measure the relative intensities of two frequency separated laser modes without such optics and use the relative intensity measurement to adjust the cavity length of a laser and thereby stabilize the lasing frequencies.
The frequency stabilized laser system is also suitable for interferometry applications. In particular, the system provides two stabilized frequency components with different polarizations suitable for heterodyne interferometry measurements. Moreover, in some embodiments, the system additionally provides a reference beam in which the polarizations of the two stabilized frequency components are mixed. As a result, the reference beam includes a time-varying intensity corresponding to the heterodyne frequency (i.e., the difference frequency between the two stabilized frequency components). In particular, no analyzer (e.g., polarizer) is required to mix the polarizations of the two frequency components to produce the time-varying intensity at the heterodyne frequency. Instead, the heterodyne frequency can be determined by directly measuring the intensity of the reference beam. Alternatively, the heterodyne frequency can be determined from a high bandwidth measurement by the first or second detector. The system can also be used in homodyne interferometry applications in which only a single stabilized frequency is necessary.
Other features, aspects, and advantages will be clear from the following detailed description and from the claims.